It is known, in the after treatment of power plant boiler flue gases, generally from coal fired power plants but also form power plants fired with other fossil fuels, to provide a scrubbing process in which a calcium compound such as lime or limestone is added to the scrubbing liquid or is otherwise provided in the gas stream adapted to contact the scrubbing liquid so that the scrubbing operation results in the generation of calcium sulfate dihydrate by the reaction of the calcium containing compound with sulfur compounds in the flue gases, usually sulfur oxides.
The calcium sulfate dihydrate which is thus formed can be recovered and can be usefully applied or treated for use as a building material in the form of gypsum or gypsum products.
In this technology it is known to utilize two flue gas scrubbing columns which are combined with an inlet for the flue gas and an outlet for the scrubbed gases and means for circulating a scrubbing liquid through the columns and for recovering calcium sulfate dihydrate from the scrubbing liquid.
In these two-column systems, a first of the scrubbing columns forms a first desulfurization stage in which the scrubbing is carried out in an acid state of the scrubbing liquid, i.e. a pH of the scrubbing liquid in the acid range. The other scrubbing column carries out a second desulfurization stage with the pH of the scrubbing liquid in a higher or more basic range.
The flue gas to be desulfurized is initially passed through the acid scrubbing column and then traverses the basic scrubbing column before being discharged. The acid scrubbing column is operated with air or oxygen addition and the calcium sulfate dihydrate can be extracted from the sump product of this column.
When I describe a basic scrubbing column, it should be understood that I intended thereby to describe a scrubbing column which is operated with a pH of the scrubbing liquid that is higher than the pH of the first or acid scrubbing column, even if this pH is not necessarily wholly in the basic range in the classic sense, i.e. even if the pH may be somewhat below 7.
In conventional scrubbing plants of the latter type, the two columns are disposed one behind the other in immediate succession and in practice it is possible to form the two columns as distinct stages in a single tower.
The two columns can be provided one above another in the scrubbing tower and the upper flue gas scrubbing column can be operated in the basic pH range while the lower column is operated in the acid pH range.
The scrubbing liquor from the upper scrubbing column can pass downwardly through a sludge separation stage to partly enter the lower scrubbing column. In practice, the flue gas to be desulfurized is introduced into the lower scrubbing column and passes from it upwardly through the upper scrubbing column, being removed from the top of the tower.
The desulfurized flue gas leaves the upper scrubbing column at a temperature of 50.degree. C. or less. From a point of view of environmental protection, this is undesirable since flue gases in such low temperatures cannot be discharged into the atmosphere from tall stacks. They consequently must be reheated in special apparatus at comparatively high cost with an environmental detriment in the sense that the reheaters must be operated with combustion of additional fossil fuels.
It has also been suggested to provide a heat exchanger for the incoming flue gas to enable sensible heat to be extracted therefrom and to utilize the sensible heat to reheat the desulfurized flue gas as it leaves the scrubbing tower. An indirect heat exchanger of this type is comparatively costly and the passages through which the flue gas is discharged tend to become contaminated with deposits which can be cleaned only with considerable difficulty and cost. These deposits are usually gypsum or gypsum compositions.